It has long been the goal of naval architects to design and construct vessels with adequate internal capacities and accommodations, structural strength, stability and seaworthiness when the vessel is afloat and sufficiently small resistance to economize propelling power at high speeds as evidenced by U.S. Pat. Nos. 2,185,430, 2,342,707 and 4,079,688.
Traditional surface ship monohull designs have usually been developed from established design principles and assumptions which concern the interrelationships of speed, stability and seakeeping. Such sacrifices have to be made to achieve significantly higher performance than hitherto that current practical displacement monohull surface ship speed improvements are essentially stalled.
For example, a major limitation of present day displacement hulls is that, for a given size (in terms of displacement or volume), their seaworthiness and stability are reduced as they are "stretched" to a greater length in order to increase maximum practical speed.
Traditional hull designs inherently limit the speed with which large cargo ships can traverse the ocean because of the drag rise which occurs at the "threshold speed". This is a speed (in knots) which is about equal to the square root of the ship's length (in feet). For example, a mid-size cargo ship at about 600 feet length has an economical operating speed of about 20 knots or some 4 knots below its design threshold speed. In order to achieve higher operating speeds with commercial loads, it is necessary to increase ship length and size (or volume) in proportion, or to increase length while reducing width or beam, to maintain the same size and volume, but at the expense of stability. Naval architects have long considered the problem of achieving significantly higher ship speeds, without increasing length or decreasing beam, as the equivalent of "breaking the sound barrier" in aeronautical technology.
In the nineteenth century, Dr. Froude first accurately measured and defined the phenomenon by which increased length is required for higher ship speeds because of the prohibitive drag rise which occurs at a threshold speed corresponding to a length Froude Number of 0.3. The length Froude Number is defined by the relationship 0.298 times the speed length ratio .sqroot..sub.L.sup.V, where V is the speed of the ship in knots and L is the waterline length of the ship in feet. Thus a Froude number of 0.298 equates to a speed length ratio of 1.0. According to Froude's teaching, to go faster for the same volume the ship must be made longer, thus pushing the onset of this drag rise up to a higher speed. As length is increased for the same volume, however, the ship becomes narrower, stability is sacrificed, and it is subject to greater stress, resulting in a structure which must be proportionately lighter and stronger (and therefore more costly) if structural weight is not to become excessive. In addition, while for a given displacement the longer ship will be able to achieve higher speeds, the natural longitudinal vibration frequency is lowered and seakeeping degraded in high or adverse sea states as compared to a shorter, more compact ship.
An alternative means to achieve high speed ships is the planing hull. To date, this popular concept has been limited to a very short hull form, i.e. typically no more than 100 feet and under 100 tons. Boats of only 50 foot length are able to achieve speeds of over 60 knots (a Froude Number of 2.53 or a speed length ratio of 8.5). This is possible because the power available simply pushes the boat up onto the surface of the water where it aquaplanes across the waves, thus eliminating the huge drag rise which prohibits a pure displacement boat of normal proportions from going more than about 9 knots on the same length of hull. However, at intermediate speeds of say 5 to 25 knots, before this 50 foot boat "gets onto the plane", a disproportionately large amount of power is required. If the 50 foot planing boat is scaled to the length of a frigate of 300 feet, these speeds scale to the precise range of 12 to 60 knots. Thus scaled, the power required for a 300 foot planing frigate to achieve its minimum practical speed (60 knots) would be about half a million horsepower; but currently such horsepower cannot be installed, let alone delivered in a ship of such small size and low displacement. Furthermore, the ensuing ride on this 300 foot ship would cause material fatigue as its large flat hull surfaces would be slammed at continuously high speed into the ocean waves inasmuch as it would be too slow to plane or "fly" across the waves as a much smaller planing craft would do.
Craft utilizing planing hulls have also been produced with waterjet propulsion. Due to limitations of size, tonnage and required horsepower, however, the use of a waterjet propelled planing hull vessel for craft over 100 feet waterline length or 100 tons displacement has not been seriously considered.
The planing hull incorporates, typically, a combination of very high power, flat or concave "vee'd" bottom sections, often incorporating warped surfaces, with an angular section or "chine" at the conjunction of the sides and bottom portion, necessary for clean flow separation giving enhanced aquaplaning capabilities and imparting higher stability at very high speeds. It also characteristically features an extremely lightweight structure of wood, aluminum or fiberglass.
U.S. Pat. No. 2,185,430 (W. Starling Burgess) describes one of many interpretations of this type of hull, of which the inventor claims "one and a principal object . . . resides in the provision of a hull form capable of operation at extremely high speeds." He defines a length beam ratio of 6 to 7.5, a characteristic speed length ratio of between 2.5 and 7.3, a displacement length ratio of between 47 and 51 and defines a speed horsepower formula for high speed hulls having a length of 30 to 45 ft. as: ##EQU1## where C=27,150 if V=knots per hour.
Scaled up to the largest size defined by Burgess ("about 250 feet in length"), the leading characteristics of his hull would be: Beam of 33 to 42 feet; design speed of 39 to 115 knots; displacement of 734 to 797 tons. Burgess teaches the power required for the minimum speed of 39 knots would be in the region of 90,000 Shaft H.P. at the minimum displacement of 734 tons, for a specific power of about 3.
Hull designs using the concept of hydrodynamic lift are known with regard to smaller ships, e.g. below 200 feet or 600 tons powered by conventional propeller drives as shown in U.S. Pat. No. 2,242,707. The shape of this hull is such that high pressure is induced under the hull in an area having a specific shape to provide hydrodynamic lift.
The monohull fast ship (MFS) develops hydrodynamic lift above a certain threshold speed as a result of the presence of high pressure under the aft part of the hull and also in the upper surfaces of the inlet pipes for the waterjets shown in FIG. 16. Such a hull reduces the residuary resistance of the hull in water as shown in FIGS. 11 and 14 described below. Therefore, power and fuel requirements are decreased. Since hydrodynamic lift increases as the square of the velocity, a lifting hull allows higher speeds to be achieved than a traditional hull which tends to "squat" or sink at speeds above a Froude number of 0.42 or a speed length ratio of 1.4. Working boats utilizing the MFS form are now being used at sea or in many of the world's harbor approaches. This hull form has also up to now been considered limited to certain size fast pilot boats, police launches, rescue launches and fast lifeboats, custom launches, patrol boats, and even motor yachts and fast fishing boats which range in size from 16 to 200 feet (from 2 to about 600 tons). For their size, these boats are much heavier and sturdier than the planing boats. In the speed range of 5 to 25 knots, they have a much smoother ride. They also use much less power for their size at speed length ratios lower than 3.0 than does the planing hull, and they are very maneuverable. Although it has generally been claimed by leading naval architects that the practical use of this type of hull is limited to quite small craft, such a hull has been used for a 600 ton yacht. However, it has never been contemplated for commercial or military ships of over 2,000 tons.
U.S. Pat. Nos. 2,342,707 (Troyer) and 4,079,688 (Diry) teach different interpretations of fast displacement hulls which, however, differ from the present invention in both hull-form and operational aspects.
Troyer teaches a "double-ended" boat with a lifting stern in order to combine the alleged superior seakeeping qualities of the pointed or "canoe" stern with the lifting qualities necessary to prevent such a boat from "squatting" at more than "a moderate speed", although such speed is not defined in any respect.
Whatever the capability of the Troyer hull to generate hydrodynamic lift at the stern, such a stern is specifically unsuitable for ships of greater than 600 tons displacement and an operational speed such as 40-50 knots for the present invention due to the fact that a wide transom stern (which Troyer specifically excludes in his teaching) is a fundamental requirement for the efficient installation of waterjets as taught by the present invention as discussed hereinafter. Furthermore, at the speeds for which the present invention is intended (viz: a speed length ratio of 1.4 to 3.0) a greater area of lift is required than is obtainable from the Troyer boat without recourse to excessive beam and associated increase in drag.
Since Troyer teaches no information concerning size, proportions, displacement, speed or power, or their interrelationship, the size or type of craft or purpose of his craft cannot be determined. However, he does teach a "specific form of stern design" for a "boat" with "pointed bow and stern portions".
The Troyer stern has, characteristically, a rounded or pointed plan-form, a chine or sharp angle at the conjunction of the bottom portion and sides below the waterline; and angles of deadrise at the stern which are greater than 10.degree.. In these important features it diverges from the design features set out for the present invention as discussed below.
U.S. Pat. No. 4,079,688 (Diry) also teaches a "displacement-type hull" intended to overcome "the rapid increase in wave generating drag attendant with increased speed", placing the relevant speed to his teaching as a Froude Number of between 0.6 and 1.20. He also teaches a multihull vessel. The major feature of Diry's teaching is: "a high speed displacement hull in which a substantial portion of length comprises a parallel midbody of constant and full section."
Waterjet propulsion systems which substantially reduce the cavitation and vibration problem of propeller drives are known as shown in U.S. Pat. Nos. 2,570,595; 3,342,032; 3,776,168; 3,911,846; 3,995,575; 4,004,542; 4,276,035; 4,611,999; 4,631,032; 4,713,027; and 4,718,870. To date they have not been perceived as useful for propelling larger ships, particularly at high speeds, and are deemed generally too inefficient because they require high pressure at the water inlet in the aft part of the submerged hull, rather than low pressure which generally exists at that portion of traditional large displacement hulls.
U.S. Pat. No. 4,276,035 (Kobayashi) is typical of these patents being applied to small boats. Kobayashi teaches an arrangement for the waterjets having two inlet pipes disposed in tandem, or one behind the other, along the aft part of the centerline of the boat. He specifically states that this is to obviate the possibility that waterjet inlets, placed alongside each other on either side of the centerline, might ventilate, or "rise out of the water" when the boat heels at an angle whilst turning.
The alteration of a ship's trim is the subject of U.S. Pat. No. 4,843,993 (P. Martin). However, Martin teaches the use of this for the purpose of optimizing single screw ship performance in varying depths of water.
There is an increasing need for surface ships that can transit oceans with greater speed, i.e. in the range of forty to fifty knots, and with high stability because of the commercial requirements for rapid and safe ocean transits of perishable cargoes, high cost capital goods, military strategic sealift cargoes, cargoes whose dimensions and density cannot be accepted for air freight, and other time-sensitive freight, particularly in light of the increasing worldwide acceptance of "just-in-time" inventory and stocking practices.
Today's container ships are tending towards greater size, for reduced cargo ton-mile costs, carrying up to 25,000 tons of containerized cargo at a time. This necessitates their visiting a number of ports on both sides of an ocean crossing to load and unload cargo. This is time-consuming and means that the largest ships can only undertake a relatively small number of ocean crossings per year, thus limiting the available financial turnover on their considerable investment cost.
A much faster--but smaller--ship, operating at between 40 and 50 knots, can undertake a transatlantic roundtrip each week between only one port on each side of the ocean crossing. Although carrying only up to 10,000 tons of cargo, this smaller, faster ship could transport about 60% more cargo per year than the larger ship, with each container being subject to a much more controlled collection and delivery system using more disciplined intermodal techniques because at each port the ship is fully unloaded and reloaded. Thus the time taken from pick-up to delivery of each container (door-to-door) could be significantly reduced. For this service a cost premium may be charged, such as is presently charged for airfreight, lying somewhere between the current sea and airfreight tariffs. This premium, together with the much greater cargo turnover on each ship, more than compensates for the increased fuel consumption required for operating at over twice the speed of most current larger container ships.
For the reasons already given, it is impracticable to achieve such an increase in speed by the traditional method of making such container ships very large because, as their length is increased to raise their threshold speed according to Froude's laws, their cargo payload and stability are eroded. Serious questions also arise over the ability of propellers to deliver the necessary power due to their performance being degraded by the onset of cavitation, their impractical size and the problems of optimizing blade pitch at intermediate speeds, which could necessitate very complex gearboxes.